The present invention relates to a method and an apparatus for trace analysis of organic compounds. The invention is particularly effective for analysis of organic compounds adhering to a silicone wafer, an electronic substrate,or the like.
It is a common practice to wash out organic compounds, such as dirt and microorganisms, that are attached to the surface of a silicone wafer or its plastic package with a chlorofluorocarbon, such as FREON or a substitute therefor. Such a washing process, however, presents the problem of a residual solvent. Even if it is washed with pure water, a very small quantity of the organic compounds are prone to remaining. Furthermore, in case of an electronic substrate, the problem of evaporation of a volatile solvent often arises during operation of the apparatus in which the electronic substrate is set, because the heat generated by the operation of the apparatus causes the evaporation of the solvent.
Conventionally, such an organic compound cannot be measured, because its density is so low that it would need to be represented in terms of ppb or ppt. Therefore, such a measurement is not conducted in reality. Even if it were to be measured, it would be extremely difficult and would have to be done manually.
Adhesion of organic compounds to the surface of a silicone wafer or the like is a major cause of defective products among semiconductor products, such as IC""s and LSI""s. As these semiconductor products are mass-produced automatically, defective products sometimes result in a tremendous loss.
In order to solve the above problems by providing a means of analyzing such a small quantity of organic compound, the applicant of the present invention had previously developed a method and an apparatus for trace analysis of organic compounds adhering to a silicone wafer or a similar object, wherein a completely automatic analysis is performed once a test piece is set in the apparatus. The applicant filed patent applications for said method and apparatus under Patent Applications Ser. Nos. 1992-88105 and 1993-97243.
Although the aforementioned invention is very effective as a means of trace analysis of organic compounds, it presents a problem in that the gas in the apparatus flows only in one direction, from the flow cell toward the valves. Due to this feature, there is the possibility of such undesirable elements as air, the blank gas, or interfering particles entering the line when the flow cell is open or in other occasions. It is prone to causing a serious problem particularly when the apparatus is used at high temperature, such as a case where the apparatus is used for analysis of a substance that volatizes at high temperature, because the air entering the system causes oxidation of communication passages or other parts of the apparatus and sometimes necessitates replacement of the entire passage. The above invention presents another problem in that it is not capable of completely protecting the communication passages or other parts from contamination by various substances during hot washing of the flow cell, which is typically performed after using a dirty test piece.
Accordingly, the present invention relates to a method of trace analysis of organic compounds which calls for setting a test piece in a flow cell, removing organic compounds from the test piece by heating it at high temperature in an oven, concentrating the compounds in a trap tube and removing the compounds from the trap tube, carrying the organic compounds into a concentrating/inlet unit, such as a thermal desorption cold trap injector, to perform cryofocusing of the compounds, and then introducing the organic compounds into a gas chromatograph to be analyzed therein, wherein a part of the purified inert gas is caused, except during the process of trapping the organic compounds, to flow into the flow cell from the direction opposite the direction from which the gas flows along the trapping line. As an apparatus to be used for said method, the invention also provides an analyzer that comprises a flow cell, a trap tube connected to the flow cell, a detector connected to the trap tube, and a flow rate controller connected to the flow cell, which can easily be brought into communication with said trap tube and is so formed as to permit a test piece, such as a wafer, to be removably set therein and also permit a carrier gas to flow therethrough, wherein a channel for purified inert gas is provided between the flow rate controller and the flow cell and also between the flow rate controller and a valve that communicates with the flow cell. According to another feature of the invention, the aforementioned channel for the inert gas is connected to a plurality of flow cells, and/or the channel for the inert gas is connected to a plurality of trap tubes.